1. Field of the Invention
This invention relates to a sheath heater, particularly to the improvement of the seal portions at the ends of a protector pipe.
2. Description of the Prior Art
A conventional sheath heater comprises a metal protector pipe open at both ends, thereof an electrically heat radiant body provided along the central line of the protector pipe, and an electrically insulating powder of a high melting point loaded in the protector pipe to fix the heat radiant body. Alumina or molten magnesia powder, generally used as the insulating powder, absorb moisture when exposed to open air, resulting in a decrease in the insulating property. To prevent the problem, the open ends of the protector pipe are sealed with silicone rubber, silicone resin, epoxy resin, glass, or the like. Electric terminals are provided in a penetrating manner through the sealings so as to contact the radiant body.
Resin used for the sealing is not satisfactory in air tightness and mechanical strength. In addition, it becomes brittle with age. This causes cracking and peeling of the resin, presenting free spaces between the resin seal and the protector pipe or the electric terminal. Glass also presents a cracking problem. Generally, a molten glass is poured at the ends of the protector pipe to form sealings, resulting in thicker glass layers. As known well, a thick glass tends to be cracked when subjected to repeated temperature variations. The cracking or peeling of the sealing material allows the insulating powder to be exposed to open air and absorb moisture, resulting in breakdown of the powder, although the powder is heated prior to use at 500.degree. C. to 700.degree. C. for removing water absorbed by and chemically combined with the powder. Attention should also be paid to the fact that the electrically insulating property of glass is impaired at high temperatures.
In order to improve the drawback inherent in the sealing with resin or glass, there has been proposed a method in which a mixture of large grains of oxide having a high melting point and fine frits of a low melting point is loaded at the ends of the metal protector pipe and then to make the mixture an integral body. This method, however, is defective in that the fused sealing material comes to contain numerous fine cells. Thus, the reinforcing effect of the high melting oxide is greatly impaired. In addition, the method of fusion fails to exhibit satisfactory effects on preventing cracking and peeling of the seal formed. Being as such, a further improvement has been called for.